The interferon system is one of the natural defenses of animal and human cells against viral infection. Interferons are produced in cells after viral infection or some other stimuli. They are excreted, react with other cells and make these inefficient in supporting the replication of a broad range of viruses. No interferon has been purified to homogeneity, and the mechanism of interferon action has not been definitely established. We have been working on the isolation of pure mouse interferon from Ehrlich ascites tumor cells induced with Newcastle disease virus. We are proposing to study the mode of interferon action in blocking virus replication. One of the processes of virus replication which is apparently a target of the interferon system is the translation of viral messenger RNA. This is manifested in extracts from interferon-treated cells as an impairment of their capacity to translate added messenger RNA. The impairment is due, at least in part, to one or more inhibitors which are loosely bound to ribosomes. We will attempt to isolate the inhibitor(s) and elucidate the way in which they block protein synthesis. Studies on the effect of the interferon system on other processes, including viral RNA synthesis, are also continued.